Systemic lupus erythematosis (SLE) is an autoimmune disease characterized by loss of tolerance to nuclear antigens. Mice deficient in Lyn, a negative regulator of B and myeloid cell function, develop SLE-like disease. Reduced dosage of Btk in B cells and lack of Btk expression in myeloid cells prevents plasma cell accumulation and autoantibodies in lyn-/- mice while maintaining B cell hypersensitivity to BCR crosslinking. This suggeststhat 1) control of plasma cell numbers by the balance of Lyn and Btk signals is a checkpoint that normally prevents autoimmunity and 2) myeloid defects may contribute to autoimmunity in lyn-/- mice. The mechanism for the Btk-dependent increase in splenic plasma cells in lyn-/- mice will be defined in Specific Aim 1. Plasma cell lifespan will be measured in vivo to determine whether increased production or increased survival of plasma cells occurs in lyn-/- mice. In vitro culture systems will be used to compare the ability of wild type, lyn-/-, and Iyn-/-Btklo plasma cells to differentiate and survive alone, in response to various stimuli, and in the presence of wild type, lyn-/-, and Iyn-/-Btklo myeloid lineage cells. In Specific Aim 2, mixed bone marrow chimeras and mice with conditional deletion of lyn in either the B or myeloid lineage will be used to determine whether autoimmunity requires Lyn deficiency in B cells, myeloid cells, or both. Specific Aim 3 will determine whether reduced Btk dosage and the Slesl suppressor allele (characterized in detail in Project 1) suppress autoimmunity via similar or different mechanisms. Iyn-/-Sles1 mice will be generated and compared to lyn-/- and Iyn-/-Btklo mice in terms of autoimmune phenotypes, splenic cell populations, sensitivity of B and myeloid lineage cells to activation, and gene expression profiles. In addition, the effect of Btk and Slesl on tolerance checkpoints in immature B cells will be compared. These studies will identify potential new therapeutic targets for SLE at the level of cell populations, pathways,and individual molecules.